This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
By buying the book, you also help to support the Lua project.

1.3 – Some Lexical Conventions

Identifiers in Lua can be any string of letters, digits, and underscores, not beginning with a digit; for instance

    i      j       i10      _ij
    aSomewhatLongName    _INPUT
You should avoid identifiers starting with an underscore followed by one or more uppercase letters (e.g., _VERSION); they are reserved for special uses in Lua. Usually, I reserve the identifier _ (a single underscore) for a dummy variable.

In Lua, the concept of what is a letter is locale dependent. Therefore, with a proper locale, you can use variable names such as índice or ação. However, such names will make your program unsuitable to run in systems that do not support that locale.

The following words are reserved; we cannot use them as identifiers:

    and       break     do        else      elseif
    end       false     for       function  if
    in        local     nil       not       or
    repeat    return    then      true      until
Lua is case-sensitive: and is a reserved word, but And and AND are two other different identifiers.

A comment starts anywhere with a double hyphen (--) and runs until the end of the line. Lua also offers block comments, which start with --[[ and run until the corresponding ]]. A common trick, when we want to comment out a piece of code, is to write the following:

    print(10)         -- no action (comment)
Now, if we add a single hyphen to the first line, the code is in again:
    print(10)         --> 10
In the first example, the -- in the last line is still inside the block comment. In the second example, the sequence ---[[ does not start a block comment; so, the print is outside comments. In this case, the last line becomes an independent comment, as it starts with --.